Continuous suspension of crystallizing solids and supersaturated liquid is very important for growing uniform crystals. A direct contact crystallizer is used to prevent fouling on heat transfer surfaces. Early designs of direct contact crystallizers used high volumes of refrigerants injected through spargers or distributors. Examples of refrigerants suggested for use in direct contact crystallizers include freon, water, alcohol solution, butane, propane, and air. However, these crystallizers have a very limited operating temperature range and have failed to provide true solid suspension or thorough mixing.
The only commercially operated direct contact crystallizers use mechanical units to recompress refrigerants such as propane or butane. These mechanical-type crystallizers are equipped with agitators for vigorously stirring the slurry and keeping the crystals in suspension. The speed of the agitator must be fast enough to prevent large crystals from settling and coagulating in the bottom of the vessel. However, fast rotating agitators can break down large crystals on impact. In addition to breaking down large crystals, the impact of crystals on the mechanically agitated surfaces of other crystals promotes secondary nucleation. Secondary nucleation is the cause of excess fine crystals which are difficult to filter and are easily caked, thus, requiring recycling.
It is known that a cryogenic liquid or gas is able to provide a large amount of refrigeration to a crystal slurry. The other advantage of a cryogenic fluid such as liquid nitrogen is that it is inert and will not contaminate the crystal slurry. However, it has not heretofore been possible to use such cryogenic fluids with conventional crystallizers. In conventional direct contact crystallizer practice, gas or liquid coolant is passed into a solution through a distributor which distributes the coolant over a wide area causing the coolant to bubble throughout the solution. By means of such coolant distribution, effective heat transfer with the solution is attained. The conventional practice of coolant introduction through a distributor, while effective in conventional crystallization practice, is inadequate if a cryogenic gas or liquid were to be employed as the coolant. In theory, holes are drilled uniformly on a distributor surface so that gas bubbles of the same size will distribute evenly over a wide body of liquid. However, it is not possible to manufacture a truly uniform distributor in practice. As a result, more gas will flow through larger holes where the pressure drop is lower. Therefore uniform gas velocity on each of the distributor holes cannot be maintained. Fluctuation of gas pressure due to the release of each individual gas bubble from the holes makes the situation more complicated. Subsequently, liquid will flow back through those apertures of the distributor where the gas velocity is lowest. This has not been a major problem in conventional practice other than experiencing some loss in sparging efficiency and in the need for periodic clean-up. However, if a cryogenic gas or liquid, such as nitrogen at a temperature less than -200.degree. F., were to be employed as the coolant, such liquid inflow would result in freezing and fouling of those apertures of the distributor. Increasing the supply pressure to the distributor will not alleviate the problem since the larger holes will form excessively large bubbles which will result in unacceptably poor heat transfer. A vaporizing cryogenic fluid will expand hundreds of times in volume. Supply pressure usually fluctuates substantially with a cryogenic fluid. A distributor will not be able to control such a wide fluctuation in pressure thus resulting in an efficiency loss.
Accordingly, it is an object of this invention to provide a direct contact crystallizer and crystallization method which will enable one to effectively employ a cryogenic fluid as the coolant.